[BC] GPS

[Mike McCarthy]

I don't see how going out 30+ miles makes it any more accurate for a proof. 
For the purpose of discussion, this example presumes a square box of 90 
ft./second lat. and lon.  Which it is around at the 38-40 deg. N parallels. 
The farther north you go, the tighter the longitudinal interval per second. 
At 42 deg.N, the ft./second longitude is around 79 ft. Latitude interval 
remains the same distance interval at all locations.

At 10 miles...52,800 ft., the span (X) of one arc degree is around 921ft. 
or +/-460 ft.  About +/-6 geographic seconds at any latitude at the E/W 
cardinal ordinates. Since most receivers resolve to 0.5 deg, that would 
result in a displayed accuracy of something less than 450 arc ft. or +/- 
225 arc ft. or +/- 3 seconds geographic at the E/W cardinal ordinates.

Now, the rest presumes a standard circle irrespective of location.

(X = Diameter in miles x 5280, x 3.14 / 360)
Y (ft./arc second.) = X /3600

At 10 miles, Y= 3"/arc-second"  225 ft= 882 arc-seconds.  1 second 
coordinate (here) =  313 (approx.) arc seconds at the cardinal N/S 
ordinates and 392 at the E/W ordinates. Better than 0.1 degree accuracy and 
PLENTY of precision when the center of the arc-degree is located at 10 
miles...or even 5 miles for that matter. NOTE, the farther north or south 
you go from a perfectly square box, the arc-seconds/longitude-seconds ratio 
will decrease or increase respectively.

At a distance of 10 miles, you're well beyond anything the FCC has ever 
remotely required for accuracy and precision, thus GREATLY diminishing any 
returns for the time/fuel invested to drive 4+ times the mileage of a 10 
mile radius. If anything, the greater distance makes it harder to locate 
the radial center since you need to cover more than 1/2 mile and then back 
track a greater distance to find the true center of the arc degree. Which 
could be as many as 10 seconds on a typical receiver. Granted the precision 
will be something less than 100 arc-seconds at that distance using a 
precision GPS, but the basic question is really WHY? The typical GPS can't 
resolve anything tighter than 30 ft. anyway..which is critical at closer in 
points.

Also, unless your GPS can resolve to 1/100 of second, you'll never have a 
tighter box than a 10 ft. square (at this latitude).  Which is the error 
margin at 1 mile radius typically on DGPS enabled RX and negates the 
"precision" of using a really distant end point.

With that said, a 30 mile end point is appropriate for longer distances, 
such as when running conductivity measurements. It allows a tighter cone to 
locate a point and be "on" the true radial at greater distances.  But it 
still does nothing for proof accuracy/precision.

MM

At 09:23 PM 2/3/2009 -0500, Cowboy wrote
>On Tuesday 03 February 2009 04:13 pm, towers at mre.com wrote:
> >  You are correct in your intitial interpritation. We drive the circumfrence
> >   around the site at a distance of 10 miles nailing the radial lines in
> >  advance. Then locate the points based on not only the distance, but
> >  location to/on the radial line quite precisely. We don't plan the route
> >  precisely prior to the process.
>
>  The further out you go, the more accurate, and the more expensive
>  the approximation will be, but "good enough" for most purposes.
>  It's still limited to the bearing display resolution of the GPS, which 
> I've yet
>  to see a consumer grade better than 1/2 degree even though most
>  can resolve mush more accurately internally.
>
>--
>Cowboy